Method of separating wood chips from bark chips



May 1, 1962 A. WESNER 3,032,188

METHOD OF SEPARATING wooa CHIPS FROM BARK CHIPS Filed Feb. 16, 1959 Partial vacuum I pplied to water containing mixed "9 Bark and wood u-"nun chips immersed Vacuum released in water livered to float- FLOAT- SINK SEPARATION Wood chips off bottom INVENTOR. ADAM L. WESNER Y B fi e ATTORNEYS United States Patent Ohio Filed Feb. 16, 1959, Ser. No. 793,564 8 Claims. (Cl. 209- 4) This invention relates to a method for sorting wood chips from bark chips. More particularly, it is concerned with this separation of wood chips made from southern pine species.

The increased demand for wood chips in the pulp and paper industry has resulted in a need for utilizing saw mill waste as an added supply of wood fiber. The saw mill waste referred to is in the form of slabs and edging strips which result from sawing a log into usable lumber. Although there are machines that have been designed to debark slabs, the need still exists for a more economical and more efiicient method of producing barkfree chips. Inasmuch as the slabs or other wood source must ultimately be chipped and screened to produce chips of a fairly uniform size, it is an object and feature of the present invention that the wood be chipped with the bark attached in such a manner as to produce free wood chips and free bark fragments and that thereafter a simple and economical method be used to sort the bark from the wood chips.

It is the principal object of this invention therefore to provide a simple, efficient, and economical method for sorting bark fragments from wood fragments.

According to the present invention, the produced mixture of wood chips and bark chips is immersed in a liquid bath, a vacuum is applied to the liquid bath in a manner that will induce, upon release of the vacuum, a substantial difference in specific gravity between the bark chips and the wood chips, the vacuum is released, and thereafter the wood and bark chips are separated by float-sink separation in a liquid having a specific gravity intermediate between the resulting specific gravities of the bark and the wood.

Prior to any type of treatment, both bark and wood usually float in water and similar liquids. By the method of the present invention, it is possible to introduce water selectively into the fibers of southern pine wood chips, thus increasing the specific gravity of the wood sufliciently so that it will sink in water. Although the bark of many species is more porous than wood, it will not absorb sufficient water to increase its specific gravity enough to cause it to sink in water, due to the difference in cell structure between the bark and the wood.

According to the present invention, it has been found that when a mixture of southern pine bark and wood chips is submerged in water and this water subjected to a partial vacuum, the entrapped air is removed from the wood fibers, due to the interconnectiveness of the cells in these fibers, but not from the bark. Thus, upon release of the vacuum the wood chips absorb appreciable quantities of water, and the bark fragments do not. Consequently, during the separation process the wood chips sink, the bark chips float, and a separation is thus achieved.

In the drawings:

The FIGURE is a schematic illustration of the process of the present invention.

With reference to the FIGURE, a mixture of bark chips 5 and wood chips 6 is immersed in water 7, or a similar liquid bath. The water level must be sufficient to completely cover all of the chips. A screen or grating may be used to keep the chips submerged. Then a partial vacuum is applied to the water containing this mixture "ice of chips. After a timed interval, the vacuum is released and the chips 5 and 6 are delivered to a float-sink separator 8. In this float-sink separation, the bark chips 5 are collected off the top and the wood chips 6 are collected off the bottom. Preferred for use as the liquid in the float-sink separator is a liquid having a specific gravity of about 1.08, although water is suitable.

Experimental tests were made in the pilot plant schematically illustrated in FIG. 1. The conditions of vacuum treatment and separation were varied. Slabs of bark and wood were chipped and the size fraction less than one inch, but more than three-eighths inch, was used for separation. For other of the tests, the minimum size fraction was lowered to one-fourth inch.

Better than percent wood recovery was consistently exhibited in the experimental tests, with the bark in the sink product varying from about one percent at a water temperature of 20 C. and a time at vacuum of about one minute to about two or three percent fora water temperature of 60 C. and a time at vacuum of about four minutes. water temperatures and longer times at vacuum recovered slightly more wood than the other tests, the small gain in wood recovery is heavily outweighed by the higher bark content. Thus, the minimum water temperatures and times are preferred. Although the bark content of the wood product was slightly increased when minimum chip size of one-fourth inch was used in place of threeeighths inch, the result was that a much higher percentage of the slab was recovered as usable chips. Thus, over-all, this smaller minimum chip size is preferred. Tests made with slab lots containing a higher percentage of locked bark (bark which remains attached to the wood even after chipping) were found to also produce wood products containing higher percentages of bark. Thus, as the proportion of locked bark increases, the bark content of the wood product made by the process of the present invention increases. A vacuum of from about 55 to about 65 centimeters of mercury (cm. Hg) is-preferred in order to permit treatment in a reasonable time at or near ambient temperatures. For the higher preferred vacuums, of course, smaller time intervals of vacuum treatment sufiice. For time intervals of the order of one minute, a vacuum of approximately 60 cm. Hg is highly preferred.

Although the use of water in the chip treatment and chip separation in the present process might be most convenient, use of a heavier liquid (specific gravity 1.08) would result in wood products containing less bark. The major portion of the bark which sinks to contaminate the wood is inner bark. The inner bark which sinks in water has a specific gravity only slightly higher than water, while the wood which sinks has a specific gravity of about 1.1. Therefore, a liquid with a specific gravity of about 1.08 will float the bark and still allow the wood to sink.

It is preferred, if possible, that the slabs to be chipped for use in the present process be aged in storage prior to shipping. It was found that, as the slabs are aged in storage, the moisture content of the slabs decreases and the percentage of locked bark in the chips decreases substantially. For example, for one slab lot tested, as the slabs were aged in storage, from 15 days after sawing until 56 days after sawing, the moisture content decreased from about 40 percent to about 32 percent, and the percentage of locked bark in the chips decreased from 1.0 to 0.03 percent.

A comprehensive investigation made in the laboratory on a large number of southern pine chip samples showed that the above vacuum treatment and separation of chips with good bark-wood liberation produced wood products that contained an average of 1.5 percent bark when water Although the tests at the higher was used for the float-sink separation and 0.8 percent bark when a liquid with a specific gravity of 1.08 was used for the separation. Vacuum treatment and separation of chips with a comparatively high proportion of locked bark produced wood products that contained an average of 2.8 percent bark when water was used for the floatsink separation, and 2.1 percent bark when a liquid with aspecific gravity of 1.08 was used for the separation. The pilot plant tests, mentioned above, demonstrated that results comparable to those of laboratory separation could be achieved on a large scale. The treatment section of the pilot plant was operated on a batch basis, and the separation of the treated chips was continuous. The tests were made with two types of float-sink separators to sort the bark from the Wood. These were a dragtype separator and a drum separator. Tests made with the drag separator on chips with loose bark (less than 2 percentlocked bark) produced wood products that contained an average of 1.1 percent bark, whereas separation: with chips. with tight bark (more than 2 percent locked bark) produced wood products that contained an average of 2.9 percent bark. The maximum capacity of the drag separator used was at least 1.5 tons of chips per hour;

Experiments on pretreatment of slabs before chipping to increase the liberation of bark from wood showed that the only procedure that was beneficial in reducing the bark content of the wood product was air-drying. Soaking the slabs in water, or in a dilute caustic soda solution, increased.- the liberation of bark during chipping, but the soaking caused a large portion of the bark to sink with the wood chips during vacuum treatment and separation.

In the separation process of the pilot plant, the treated chips enter the drag separator through a revolving stargate feeder and are dispersed into the settling pool. The barkfloatsand is skimmed from the surface of the water by two multiple paddle wheels. The Wood sinks and is conveyed by. flights along the bottom and up the incline portion, where it is dewatered and discharged from the separator. A mechanical sampler operating on a timed schedule. is used to obtain representative samples of the wood chip product. Some tests performed indicated that a delay between vacuum treatment and float-sink separation had a significant adverse effect on the bark content of the wood product. The best way to eliminate this delay is by use of a continuous method of treatment with a delay between treatment and separation that is small.

Several examples of the process of the present invention follow.

Example 1 The chips fed in for the vacuum treatment and separation consisted of chipped slash pine slabs with the fraction minus one plus one-fourth inch being used; 906 percent of the chipped slab was in this fraction. The total bark content was 20.7 percent with 0.03 percent of locked bark. These chips Were immersed in water at a temperature of 20 C. and treated for one minute at a partial vacuum of 60 cm. Hg. The vacuum was then released and the treated chips were delivered to a dragtype float-sink separator, with water being used as the separating liquid. The percentage of the feed recovered in the sink product of the separator, in weight percent, was 79.5 percent. The percentage of bark in the sink product was 1.1 percent, 99.1 percent of the wood feed being recovered.

' Example] w Same slab lot as Example 1 and same conditions. The total bark in the feed chips was 17.8 percent with 0.03 percent locked bark. The percentage of the feed recovered in the sink product of the separator was 82.1 percent. The percentage of bark in the sink product was 1.1 percent, 98.8 percent of the wood feed being recovered.

Example 3 The chips fed in for the vacuum treatment and separation consisted of chipped longleaf pine slabs with the fraction minus one plus one-fourth inch being used; 90.6 percent of the chipped slab was in this fraction. The total bark content was 18.3 percent with 0.5 percent of locked bark. Same treatment as Example 1. The percentage of the feed recovered in the sink product was 78.8 percent. The percentage of bark in the sink product was 1.4 percent, 95.2 percent of the wood feed being recovered.

Example 4 The chips fed in for the vacuum treatment and separation consisted of chipped loblolly pine slabs with the fraction minus one plus one-fourth inch being used; 90.6 percent of the chipped slab was in this fraction. The total bark in the feed chips was 16.8 percent with 3.5 percent locked bark. Same treatment as Example 1. The percentage of the feed recovered in the sink product was 78.1 percent. The percentage of bark in the sink product was 2.6 percent, 91.3 percent of the wood feed being recovered.

Example 5 The chips fed in for the vacuum treatment and separation consisted of chipped loblolly pine slabs with the fraction minus one plus three-eighths inch being used; 80.1 percent of the chipped slab was in this fraction. The totalbark content was 12.6 percent with 0.03 percent of locked bark. The chips were immersed in water at a temperature of 20 C. and treated for one minute at a partial vacuum of 60 cm. Hg. The vacuum was then released and the treated chips were delivered to a drag-type float-sink separator with water being used as the separating liquid. The percentage of the feed recovered in the sink product of the separator was 83.9 percent. The percentage of bark in the sink product was 0.5 percent, 95.5 percent of the wood feed being recovered.

Example 6 Same slab lot and treatment as Example 5 except that the time at vacuum was four minutes instead of one minute, and the water temperature during vacuum treatment was 60 C. instead of 20 C. The percentage of the feed recovered in the sink product of the separator was 87.9 percent. The percentage of bark in the sink product was 1.5 percent, and 99.8 percent of the wood feed was recovered.

Example 7 Same slab lot and treatment as Example 5 except that the time at vacuum was four minutes instead of one minute. The percentage of the feed recovered in the sink product of the separator was 82.7 percent. The percentage of bark in the sink product was 0.9 percent, and 95.9 percent of the wood feed was recovered.

Example 8 Same slab lot as Example 2 and same chip size and treatment as Example 5. The percentage of the feed recovered in the sink product of the separator was 82.7 percent. The percentage of bark in the sink product was 1.3 percent, and 97.2 percent of the Wood feed was recovered.

Example 9 Same slab lot, treatment, and conditions as Example 8, except that the time at vacuum was four minutes instead of one minute, and the water temperature during vacuum treatment was 60 C. instead of 20 C. The percentage of feed recovered in the sink product of the separator was 86.0 percent. The percentage of bark in the sink product was 2.7 percent, and 99.6 percent of the wood feed was recovered.

Example Same slab lot, treatment, and conditions as Example 4, except that the water temperature during vacuum treatment was 50 C. instead of 20 C. The percentage of the feed recovered in the sink product was 78.2 percent. The percentage of bark in the sink product was 4.9 percent, and 92.2 percent of the wood feed was recovered.

What is claimed is:

1. A process for separating a mixture of southern pine wood chips and southern pine bark chips which comprises: immersing said mixture in a liquid bath; applying a vacuum to said liquid bath in a manner sufiicient to induce, upon release of said vacuum, a substantial difference in specific gravity between said bark chips and said wood chips; releasing said vacuum; thereafter delivering said bark chips and said wood chips into a liquid having a specific gravity intermediate and between the specific gravities induced in said bark chips and said wood chips; and collecting wood chips substantially free from bark chips from a lower region of said liquid.

2. The process of claim 1 wherein the specific gravity of the chips which is induced is less than about one for the bark chips and greater than about 1.08 for the wood chips.

3. The process of claim 2 wherein the wood chips and bark chips used are sized more than about one-fourth inch and less than about one inch.

4. A process for separating southern pine wood chips from southern pine bark chips which comprises: immersing a mixture of said wood chips and said bark chips in a liquid bath whose temperature is less than about 60 C.; applying at least about 60 centimeters mercury vacuum to said liquid bath for a time of less than about four minutes; releasing said vacuum; thereafter delivering said bark chips and said wood chips into a liquid having a specific gravity of from about 1.0 to about 1.08; collecting bark chips off the surface of said liquid; and collecting wood chips substantially free from bark chips from a lower region of said liquid.

5. The process of claim 4 wherein the wood chips and bark chips are sized more than about one-fourth inch and less than about one inch.

6. A process for separating southern pine wood chips from southern pine bark chips which comprises: immersing a mixture of said wood chips and said bark chips in a liquid bath; applying about centimeters mercury vacuum to said liquid bath for a time of from about one to about four minutes; releasing said vacuum; thereafter delivering said bark chips and said wood chips into an aqueous liquid having a specific gravity of from about 1.0 to about 1.08; collecting bark chips off the surface of said aqueous liquid; and collecting wood chips substantially free from bark chips from a lower region of said aqueous liquid.

7. The process of claim 6 wherein the liquid bath is at a temperature of approximately 20 C.

8. The process of claim 7 wherein the vacuum is applied to the liquid bath for about one minute.

References Cited in the file of this patent UNITED STATES PATENTS 

